Oxidation of unsaturated fats and oils is essentially a degradation process which occurs at the double-bond sites in glyceride molecules--the building blocks of edible fats and oils. The process proceeds via what is commonly referred to as a "free-radical" mechanism in which the initiation step is the formation of a fatty free radical when hydrogen departs from the .alpha.-methylenic carbon in the unsaturated fatty acid group of the oil molecule (RH). The resultant free radical (R.) becomes very susceptible to attack by oxygen to form an unstable peroxide free radical (ROO.). These free radicals themselves serve as strong initiators and promoters (catalysts) of further oxidation, hence oxidative breakdown of fats and oils becomes a self-perpetuating (autocatalytic) process, giving rise to a chain reaction. Auto-oxidation of highly unsaturated fatty acids can yield hard, tough, insoluble polymeric products. In the final or terminating stage of glyceride oxidation, hydroperoxides that form split or decompose into short-chain aromatic compounds (mainly aldehydes, ketones, alcohols, and acids), which cause the rancidity condition that ultimately destroys acceptability and usefulness of fats and oils.
Auto-oxidation is initiated or strongly catalyzed by a number of factors. Heat, for instance, greatly accelerates oxidation, especially at higher temperatures (above 60.C) where it has been estimated that for each 15.degree. increase in temperature, the rate of oxidation reaction doubles. Another important factor is the presence of metals, which in just trace amounts are recognized as the predominant prooxidant materials encountered in commercial fats and oils. It is estimated that copper or iron at concentrations of less than 1 ppm can cause very serious reduction of fat and oil stability. This problem is magnified by free fatty acids which act to solubilize metals in fats or oils. Sherwin, E. R., "Oxidation and Antioxidants in Fat and Oil Processing", JAOCS, 55, 1978 (809-814).
Since the glyceride auto-oxidation is initiated and promulgated by the formation of free radicals, it follows that removal or inactivation of the fatty or peroxide free radicals should terminate, or at least interrupt, fat oxidation in its early stages, and thus delay breakdown into the final end products that are responsible for rancidity.
Antioxidants such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) are widely used in many foods to prevent fat rancidity. These compounds are added at concentrations ranging from 50 to 200 ppm to suppress the development of peroxides during food storage. The antioxidative activities of these synthetic substances stem from the phenolic configuration of their molecular structures. These phenolic substances function as free radical acceptors which can terminate fat or oil oxidation at the initiation stage. The antioxidant free radical that forms is stable and, most importantly, does not initiate or promulgate further oxidation of the glyceride.
It has been long recognized that tocopherol molecules, which have the phenolic configuration, exhibit antioxidant properties. There are at least seven types of tocopherol with the .alpha., .gamma. and .delta. isomers predominating in vegetable matter. Vitamin E activity is attributed mainly to .alpha.-tocopherol, which also provides some oxidation inhibition effect in oil, but the .gamma. and .delta. forms are more effective antioxidants. Tocopherols are widely distributed in many vegetable matters from which commercial edible oils are extracted. Moreover, a high proportion of these "natural" antioxidants survive commercial oil processing to end up in the finished oils at levels as high as 500-1,000 ppm. These "residual" tocopherols are largely responsible for the oxidative stability inherent in finished vegetable oils. However, it is recognized that these tocopherol levels seem to be optimum for providing oxidative stability, and that addition of more will likely provide no further improvement in stability. Indeed, the addition of tocopherols to oils may even have a depressing effect on the oxidative stability of the oil, but further improvement in stability of finished oil may be achieved through addition of one or more of the approved synthetic oxidants. Sherwin, supra, at p. 813. One explanation is that the naturally present tocopherols mask additional activity. Cort, W. M. "Antioxidant Activity of Tocopherols, Ascorbyl Palmitate, and Ascorbic Acid and Their Mode of Action", JAOCS, 51:7, 1974 (321-325).
Besides BHT, BHA and tocopherols, there are a variety of other "primary" antioxidants which also function by inhibiting or interrupting the free radical mechanism of glycerine auto-oxidation. In addition, it has been long recognized that various acids (both organic and inorganic), and some of their derivatives, provide apparent antioxidant effect when added to vegetable oils. These are commonly referred to as acid-type antioxidants. However, these acids, if added alone to oils containing no primary antioxidant, will exhibit virtually no effect on the oxidative stabilities of the oil. It is believed that the acids are not truly antioxidants but more likely function by enhancing, in some manner, the activity of primary antioxidants naturally present (such as tocopherols) in the oils, or those antioxidants that are added. Common acid-type antioxidants include citric, phosphoric, thiodipropionic, ascorbic, and tartaric acids. Because these acids are generally insoluble in vegetable oils, they cannot be used directly with the primary antioxidants. Instead, it has been found that certain derivatives of these acids are effective. These include isopropyl citrate, didodecyl thiodipropionate, dilauryl thiodipropionate, dioctadecyl thiodipropionate, and ascorbyl palmitate. Sherwin, E. R. "Antioxidants for Vegetable Oils", JAOCS, 53, 1976 (430-436).
Unlike the primary antioxidants which function as electron donors, ascorbic acid and ascorbyl palmitate function by oxygen scavenging, an entirely different mechanism. Ascorbyl palmitate can be weighed directly into oils, dissolved in ethanol, and added to the oils or dissolved in a special oil, such as decaglycerol octaoleate. With the last method, solubility of 0.05 percent in oils can be achieved.
It has been found that tocopherols exhibit antioxidant activity in animal fats; moreover, their activity is enhanced by ascorbyl palmitate. Furthermore, with vegetable oils, addition of ascorbyl palmitate improves the shelf lives of the oils, although no further improvement is seen by the addition of tocopherols to vegetable oil. It would appear that the residual tocopherols in the vegetable oil is at the optimum level, and that antioxidant activity is not enhanced by adding more. Cort, supra, pp. 323-325.
A number of compounds have been isolated from herbs and spices which exhibit antioxidant activity. Carnosol and rosmanol, both phenolic diterpenes have been isolated from rosemary. More recently, another antioxidant, rosmaridiphenol, was isolated. The antioxidant activity of rosmaridiphenol surpasses BHA and approached the effectiveness of BHT. Houlihan et al., "Elucidation of the Chemical Structure of a Novel Antioxidant, Rosmaridiphenol, Isolated from Rosemary", JAOCS, 61:6, 1984 (1036-1039). However, the basic component of rosemary is borneol, which exhibits only slight antioxidant activity due to the absence of aromaticity. Farag, et al., "Antioxidant Activity of Some Spice Essential Oils on Linoleic Acid Acidation in Aqueous Media", JAOCS, 66:6, 1989 (792-799). It is believed that the antioxidant activity of rosemary extracts is primarily related to its carnosic acid and carnosol content. Moreover, rosemary extracts have been shown to have antioxidant activity comparable to BHA and BHT. Bracco et al., "Production and Use of Natural Antioxidants", JAOCS, June 1981 (686-690).
As consumers become increasingly conscious of the nutritional value and safety of their food and its ingredients, questions have been raised regarding the desirability of using synthetic antioxidants. There continues to be a critical need for effective natural antioxidants for use with vegetable oils. There is a need for natural antioxidants that remain effective even in the presence of minerals and metals.